Recently, from a viewpoint of the weight reduction of the vehicle body in the automobile, it is demanded to increase the strength in the hot rolled steel sheets which are used in the structural part of the automobile, underbody parts such as a wheel, a rim and a chassis, high-strength parts such as a bumper and a door guard bar, and so on. Above all, such a demand is particularly strong for high-strength steel sheets having a tensile strength of not less than 590 MPa. In addition, the hot rolled steel sheets used in such applications are required to have a good anti-fatigue property. Especially, the underbody parts supporting the weight of the vehicle body are required to have an excellent anti-fatigue property in the bending mode because a large bending deformation is applied to the steel sheet.
In general, as the high-strength steel sheet is high in the yield point and easily causes the springback during the forming, it is considered to hardly provide a given shape by a press work. In order to solve such a problem, therefore, JP-A-55-28375 proposes a steel sheet having an improved shape fixability in which it is made possible to lower the yield point as compared with the degree of the tensile strength by dispersing hard martensite into soft ferrite to form a dual phase microstructure.
However, it is lately desired to further improve the press formability in order to properly cope with the high-strengthening of the steel sheet for the weight reduction of the vehicle body, the common die forming in the parts constituting a vehicle body, the complication of the shape of the parts and the like.
As the press formability is affected by the surface roughness to no small extent, it is examined to adjust the surface roughness to improve the press formability.
A technique for improving the press formability by properly adjusting the surface roughness of the steel sheet as mentioned above is disclosed in, for example, JP-A-6-99202. This technique ensures good frictional characteristics and improves the press formability by adjusting the surface roughness, which is provided by the control of a skin pass rolling, in accordance with the strength of the steel sheet with respect to thin steel sheets produced by the continuous annealing.
However, the technique disclosed in JP-A-6-99202 targets steel sheets having inherently a small surface roughness such as cold rolled steel sheets and surface treated steel sheets, so that there is a problem that it is difficult to apply the above technique to steel sheets having inherently a large surface roughness resulted from the push-in of scale or the like during the rolling such as hot rolled steel sheets.
And also, a technique providing the hot rolled steel sheet suitable for use in applications for working and forming such as a stamping or the like by adjusting the surface roughness of the steel sheet is disclosed in JP-A-9-118918. This technique intends to improve the frictional characteristics and the ductility by rendering the surface roughness of at least one surface of the steel sheet into Ra of not more than 0.8 μm, Rmax of not more than 4.0 μm and Rv/Rmax of not more than 0.7. Moreover, the term “Rv” used herein means a distance from a deepest valley to a center line in a measured length of a profile curve.
However, as this technique intends to improve the workability only by the surface roughness, when the steel sheet obtained by this technique is subjected to the forming accompanied with a large working amount as in an inner plate of the automobile, there is a fear that the die-galling is easily caused in a portion having the large deformation quantity and the cracking is caused therewith.